The accepted standard in neutron detection has been based on helium-3 (He3). One problem with conventional neutron detectors based on helium-3 is that helium-3 is a natural resource with a very limited supply. These types of detectors and all other known neutron detectors have a gamma rejection of approximately up to 4 gamma pulses in 10,000 pulses detected. Unfortunately, these levels of gamma rejection in conventional neutron detectors can result in too many false positive alarms, indicating that a neutron particle has been detected when in reality a gamma particle was detected. Gamma particles can occur from natural phenomena, such as from the sun, while neutron particles typically indicate a presence of radioactive and/or fissile material. Accurate detection of the occurrence of the neutron particles, without false detection of gamma particles as neutron particles, is critical for monitoring border activities such as during homeland defense and security.
The need for an efficient neutron detector, with little to no false positive alarms due to gamma cross-talk, is critical in many applications such as for homeland security, medical applications, and military applications.
Neutron detectors that are not based on helium-3, generally use lithium 6 or boron 10 dissolved uniformly into a plastic or glass scintillator. One problem with these types of detectors is that they produce much less light per neutron collision event and require much more gain in a photomultiplier tube (PMT). These types of devices also have increased gamma ray sensitivity and use analog techniques to separate gamma from neutron collision events, which typically result in gamma pulse rejection rates of 4 in ten thousand, leaving an unsatisfactory rate of gamma false positives.
An example of a lithium 6 (6Li) neutron detector is described in U.S. Pat. No. 7,244,947 “Neutron detector with layered thermal-neutron scintillator and dual function light guide and thermalizing media” filed on Apr. 13, 2004 by Polichar and Baltgalvis and issued on Jul. 17, 2007. They describe a broad spectrum neutron detector with a thermal neutron sensitive scintillator film interleaved with a hydrogenous thermalizing media. In the Polichar invention, lithium 6 material is combined with Zns (Ag) material and a hydrogenous binder to form a thermalizing neutron detector layer. The neutrons collide with the scintillation layer to create light that is transported to a photosensor. Moderator materials applied between the neutron detector layers thermalize the neutrons. The phosphor and fiber optics both act as efficient gamma detectors.
The 6LiZns(Ag) neutron detector described in U.S. Pat. No. 7,244,947 is described in detail in a Bicron Corporation and the Los Alamos National Laboratory (LANL) report published and released to the public in 2002: “Prototype Neutron-Capture Counter for Fast-Coincidence Assay of Plutonium in Residues”. The Bicron/LANL team described the use of 6LiZnS(Ag) mixture in a hydrogenous binder (moderator material) for each detector layer. Both Polichar and the Bicron/LANL team acknowledge gamma interference that occurs with this type of detector and their need to find a method to separate the neutron and gamma signals.
Analog pulse shape differentiation was discussed as a possible means to address the gamma interference. However, the analog pulse shape differentiation methods available were technically insufficient to correct the gamma interference. The neutron detection efficiencies, per layer, and the gamma interference rates described in the Polichar invention and the Bicron/LANL team report require significant improvements to become a viable product that can compete with conventional neutron detector technologies such as the 3He neutron detector. In addition, the use of moderator material within the 6LiZnS(Ag) detector mixture or between the 6LiAnS(Ag) detector layers causes a loss of thermal neutrons due to absorption by the moderator material reducing the number of available thermal neutrons for detection.
Furthermore, a thesis was published by Mr. Thomas McKnight describing the 6LiZnS(Ag) multi-layer detector using a hydrogenous binder. Again, the neutron detection efficiencies, per layer, and the gamma interference rates described in the McKnight thesis require significant improvements to become a viable product that can compete with conventional neutron detector technologies such as the 3He neutron detector. The McKnight design also uses moderator material within the 6LiZnS(Ag) detector mixture reducing the number of thermal neutrons available for detection due to absorption by the thermalizing moderator material. Analog pulse shape differentiation is discussed as a possible means to address the gamma interference. However, the analog pulse shape differentiation methods available were insufficient to correct the gamma interference.
Current attempts at the detection of special nuclear materials such as highly enriched uranium have had difficulties with the low number of neutrons and the ability to shield low gamma energy that are generated from these materials. Those gamma detectors that can identify highly enriched uranium rely on low energy gamma below 200 Key, which can be easily shielded. Therefore, conventional detectors do not adequately detect special nuclear materials.